Blender Add-on: Import-Export Three.JS doesn't activate
The warning on the UI



is only telling you this is an Exporter add-on only. "Importer not included". Everyone using that version of the add-on will see that warning. It's not an error, if you look at the files in the three.js  repository (blender/addons/io_three)  you'll see that there's only an Exporter.

As for the report in the Warning panel, that's different.